Carbon-Centered Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 113

The gain in stabilization by substituents is compared for radicals and cations in
Table 6.8. For radicals the change from methyl to ethyl or even tertiary butyl is
not linked to a large gain in stabilization energy. In cations, however, alkyl and
alkoxyl substituents have a dramatic effect. Thus, the stabilizing effect of sub-
stituents on radical cations is mainly due to the stabilization of the cation and
only to a small extent to that of the radical.
Considerable stabilization is achieved by two methoxy groups, and the radi-
cal cation resulting from reaction (49) is stable with respect to its reaction with
water ( k < 10 3 s − 1 ) (Behrens et al. 1980). Its radical properties are not altered by
the stabilization of the cation, and hence these radical cations decay bimolecu-
larly at diffusion-control rates.

The methylenedioxolane radical cation is somewhat less stable and reacts with
water at a rate of 7 10 − 2 s − 1 [reaction (51)] and (as other radicals cations also do)

with phosphate ions [reaction (50)]. The radical formed in reaction (51) opens
the ring upon deprotonation [reaction (52)]. Reactions analogous to reaction
(61) can be used as a probe to trap short-lived radical cations.

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As expected, other radicals with a good leaving group in -position can give rise

to the formation of radical cations (Koltzenburg et al. 1982), but it is quite sur-
prising that radical cations can also be formed under certain conditions when
the leaving group is in the

β

-position, for example, reaction (53) (Koltzenburg et
al. 1983). The lifetime of the radical is ca. 10 − 5 s and that of its radical cation with
respect to its reaction with water has been estimated at about 10 − 8 s [using the
competition with phosphate; cf. reaction (50)].

γ